1. Technical Field
The invention relates to a diagnostic system for an internal combustion engine including a filter that is provided in an exhaust passage of the internal combustion engine and that collects particulate matter (PM) in exhaust gas.
2. Background
There is known a technique that a filter that collects PM in exhaust gas is provided in an exhaust passage of an internal combustion engine. A malfunction, such as melting damage and breakage, may occur in the filter. When such a malfunction of the filter occurs, the amount of PM that is not collected by the filter and that flows out from the filter increases. Such a malfunction of the filter leads to an increase in the amount of PM that is released to the atmosphere. There has been developed a technique for detecting a pressure difference in exhaust gas between the upstream side and downstream side of a filter (hereinafter, which may also be referred to as “filter differential pressure”) and then diagnosing on the basis of the filter differential pressure whether there is a malfunction in the filter.
There is also known a configuration that a urea water addition device and a selective reduction NOx catalyst (hereinafter, which may also be referred to as “SCR catalyst”) are provided in an exhaust passage of an internal combustion engine. With this configuration, urea added from the urea water addition device hydrolyzes to produce ammonia. NOx in exhaust gas is reduced in the SCR catalyst by using the ammonia as a reducing agent. Japanese Patent Application Publication No. 2010-121478 (JP 2010-121478 A) describes that, with the above configuration, a deposit originated from urea water added from the urea water addition device (hereinafter, which may also be referred to as “urea deposit”) may accumulate in the exhaust passage. Japanese Patent Application Publication No. 2011-220232 (JP 2011-220232 A) describes means for estimating the accumulation amount of urea deposit in an exhaust system.